Hi @BETANCUR Natalia (ENGIE North America) ,
As per my test, this issue cannot be reproduced from my end. I have assigned the contribute permission to a user, and he can use the @comments feature when entering a username. The following screenshot is result of this test.

Before moving forward, we need more detailed information about the situation you are experiencing.
- Have you check this user permission in the Site and in the Lists?
- Does other users experiencing the same behavior? You may try the following suggestions below.
- Kindly try InPrivate browsing and check if still the same behavior.
- Kindly check if the item you're trying to add a comment do not have unique permission.
- Click on the ellipses button > Manage access > Scroll down and click on Advanced.

If still the same behavior after using these suggestions, I would suggest you raise a ticket Get support - Microsoft 365 admin | Microsoft Learn our Support Engineer will help you to capture logs to analyze the behavior.
Hope the suggestion can help you.
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